Expanded GGGGCC (G4C2) hexanucleotide repeats in the C9orf72 gene were recently identified as the most common genetic cause of Amyotrophic Lateral Sclerosis (ALS) and Frontotemporal Dementia (FTD), two neurodegenerative disorders with genetic and pathological overlap. Repeat-RNA-toxicity mediated by sequestering key RNA binding proteins is thought to play key roles in c9ALS/FTD pathogenesis. However, which RNA binding protein(s) might be sequestered by long G4C2 repeat RNAs is still unknown. We propose to identify RNA-protein interactions in c9ALS/FTD using disease-relevant repeat lengths and cell models, and by comparison to interacting-proteins of (TG3C2) repeats, a similar repeat expansion that leads to a clinically disparate disease Spinocerebellar Ataxia type 36 (SCA36). The identified RNA binding protein(s) by the G4C2 repeats will be further validated in c9ALS/FTD patient postmortem brain samples and its functions will be studied in iPS-derived motor neurons and in mice. Results from this proposal will provide new insight into the cellular cascades that cause neurodegeneration in c9ALS/FTD.